Without limiting the scope of the present disclosure, its background will be described with reference to operating a positive displacement fluid motor during downhole directional drilling operations, as an example.
In a typical downhole drilling motor, power generation is based upon the Moineau pump principle. In this type of motor design, a rotor and stator assembly converts the hydraulic energy of a pressurized circulating fluid to the mechanical energy of a rotating shaft. The rotor and stator are typically of lobed design, with the rotor and stator having similar lobe profiles. The rotor is generally formed from steel having one less lobe than the stator, which is typically lined with an elastomer layer.
In general, the power section may be categorized based upon the number of lobes and effective stages. The rotor and stator lobes are of a helical configuration with one stage equating to the linear distance of a full wrap of the stator helix. The rotor and stator lobes and helix angles are designed such that the rotor and stator seal at discrete intervals, which results in the creation of axial fluid chambers or cavities that are filled by the pressurized circulating fluid. The action of the pressurized circulating fluid causes the rotor to rotate and precess within the stator. Motor power characteristics are generally a function of the number of lobes, lobe geometry, helix angle and number of effective stages. Motor output torque is directly proportional to the differential pressure developed across the rotor and stator. Bit rotation speed is directly proportional to the circulating rate of the pressurized circulating fluid.
It has been found, however, that typical rotor and stator assemblies used in downhole drilling motors have certain maximum torque output limitations. For example, operations above a maximum differential pressure may cause fluid leakage between the rotor and stator seals which may result in no rotation of the bit due to the rotor becoming stationary or stalling in the stator. As such, in the event the drill bit becomes stuck, it is not uncommon for the torque required to free the bit to exceed the maximum torque output of conventional downhole drilling motors. In such cases, one solution has been to release the downhole drilling motor and drill assembly in the well and perform a sidetrack operation to bypass the stuck components and continue drilling the well. While this solution enables continued drilling, it is not desirable as it is time consuming and expensive.
Therefore, a need has arisen for an improved downhole drilling assembly for use in directional drilling operations. A need has also arisen for such an improved downhole drilling assembly that is capable of transmitting sufficient torque to free a stuck bit. Further, a need has also arisen for such an improved downhole drilling assembly that is capable of continued drilling operations after the stuck bit is freed.